What do we use % symbol for and why there is a comma after the string end, following a name of a variable?

Hi, the % is a special character in the printf-method. The character says to insert a variable here. So it's like a placeholder. The character following % tells the printf method what type of variable is being inserted there. In your example it says %s, so the method knows it's a String that is going to be inserted.

Here you see the difference between the printf-method and the println-method (I use the System.out instead of console):

String name = "Boba Fett";

System.out.printf("The fictional character %s is a bounty hunter from the Star Wars universe.\n", name);

System.out.println("The fictional character " + name + " is a bounty hunter from the Star Wars universe.");

They do the same, but in a slightly different manner.

Now, why does your code not work? It's because of this: String*.*firstName = "Bobby";
There should not be a dot there, only a space. When you have a dot you are usually trying to access a method (or a field) on that class or that object. But here you are simply declaring and initialising a String variable by the name of firstName.

If you have any more questions, just ask.
Good luck!